1. Field of the Invention
The present invention relates in general to active suspension systems of a type in which a pressure holding system is provided between a fluid supply device (viz., hydraulic pump) and a pressure control valve, and more particularly, to active suspension systems of a type which is designed to control the posture change of a vehicle body at the time when the fluid supply device starts to operate.
2. Description of the Prior Art
One conventional active suspension system of the above-mentioned type is shown in Japanese Patent First Provisional Publication No. 1-249506.
In this prior art active suspension system, a fluid supply device and a pressure control valve are connected through a line pressure pipe and a drain pipe. The line pressure pipe has a line pressure check valve disposed therein, and the drain pipe has a pilot control type check valve disposed therein. The pilot control type check valve assumes its closed condition when a pilot pressure applied thereto is below a predetermined level. As the pilot pressure, a line pressure between the line pressure check valve and the pressure control valve is used.
Under operating condition wherein the fluid supply device supplies the control valve with a predetermined hydraulic pressure, the pilot control type check valve is kept fully opened and thus the return side of the pressure control valve is kept communicated with a tank of the fluid supply device through the drain pipe. However, when the fluid supply from the fluid supply device is stopped, the hydraulic pressure in a primary side of the control valve is gradually reduced, and when thereafter the hydraulic pressure becomes below the predetermined level, the pilot control type check valve is turned to assume the full close position. With this closing, the drain pipe becomes closed thereby causing the line pressure check valve and the pilot control type check valve to constitute a closed hydraulic circuit of the control valve. Due to this closed hydraulic circuit, a pressure holding condition is created and thus a rapid change of the posture of the vehicle body is suppressed.
Another prior art active suspension system is shown in Japanese Patent First Provisional Publication No. 2-171313, which has an improved responsibility with respect to the system of the Publication No. 1-249506. That is, in the system of Publication No. 2-171313, an accumulator is provided to the drain pipe of the control valve for absorbing a back pressure. That is, due to provision of such accumulator, the back pressure caused by a flow resistance in the drain pipe is absorbed, which improves the responsibility of the control valve.
However, the system of the Publication No. 2-171313 has a new drawback which is caused by provision of the accumulator.
That is, when, with the pilot control type check valve being fully closed, the hydraulic circuit including the pressure control valve forms the closed hydraulic circuit to create the pressure holding condition, the hydraulic fluid is forced to flow into the accumulator. The flowing of the fluid into the accumulator causes the hydraulic pressure in the closed hydraulic circuit to become below the predetermined level of the pilot control type check valve and thus lowers the height of the vehicle body. Furthermore, due to unavoidable fluid leakage from a hydraulic cylinder, the lowering of the vehicle body height is promoted.
When, with the vehicle body being lowered in height, the fluid supply device starts to operate and the discharge pressure of the fluid supply device exceeds the holding pressure of the closed hydraulic circuit, the hydraulic fluid is fed to the closed hydraulic circuit through the line pressure check valve. However, since, under this condition, the pilot control type check valve keeps its full closed condition, the feeding of the fluid into the closed hydraulic circuit brings about a rapid increase of the pressure in the closed circuit thereby inducing a rapid lifting of the vehicle body. This makes the passengers in the vehicle feel uncomfortable.